1. Field of the Invention
The present invention relates to a mixed ionic conductor and an electrochemical device, such as a fuel cell or a gas sensor, using the same.
2. Description of the Prior Art
The applicant has long been actively developing mixed conductors of protons and oxide ions (see for example Publication of Unexamined Japanese Patent Application (Tokkai) No. H5-28820 or H6-231611). These mixed ionic conductors are basically perovskite oxides containing barium and cerium wherein a portion of the cerium has been substituted by the substitute element M, so as to achieve a high ionic conductivity (chemical formula: BaCe1xe2x88x92pMpO3xe2x88x92xcex1). Especially, when the substitution amount p of the substitution element M is 0.16 to 0.23, the mixed ionic conductor has a high conductivity, higher even than zirconia-based oxides (YSZ: yttrium-stabilized zirconia), which conventionally have been used as oxide ionic conductors. As the substitution element M, rare earth elements are suitable, in particular heavy rare earth elements, because of their atomic radius and charge balance.
New fuel cells, sensors and other electrochemical devices using such materials as a solid electrolyte have been developed. The sensor characteristics and the discharge characteristics of fuel cells using such materials have been shown to be superior to prior devices. Other patent applications related to these materials are Tokkai H5-234604, Tokkai H5-290860, Tokkai H6-223857, Tokkai H6-290802, Tokkai H7-65839, Tokkai H7-136455, Tokkai H8-29390, Tokkai H8-162121, and Tokkai H8-220060.
However, these materials show some problems with regard to their chemical stability. For example, barium tends to precipitate in CO2 gas. To solve these problems, the applicant has proposed a counter-strategy in Tokkai H9-295866. However, even this counter-strategy is not perfect, and for example at low temperatures of 85xc2x0 C. and 85% humidity, precipitation can be observed in shelf tests and boiling tests in water. Moreover, under high water vapor pressures as during discharge of the fuel cells, barium can be seen to precipitate near the platinum electrodes. Furthermore, with gas sensors, there is the problem of maintaining high ion conductivity at lower temperatures over a long time and the problem of raising the acid resistance of the oxide itself.
To solve these problems, it is an object of the present invention to improve the chemical stability of the mixed ionic conductors.
The main cause for decomposition of the oxides due to humidity is believed to be the fact that the segregated barium turning into barium hydroxide reacts with the carbon dioxide, and forms stable barium carbonate. To increase the moisture resistance, the present invention uses a mixed ionic conductor including the following perovskite structure oxide.
A mixed ionic conductor of one embodiment of the present invention (a first ionic conductor) includes an ion conductive oxide having a perovskite structure of the formula Baa(Ce1xe2x88x92bM1b)LcO3xe2x88x92xcex1, wherein
M1 is at least one trivalent rare earth element other than Ce;
L is at least one element selected from the group consisting of Zr, Ti, V, Nb, Cr, Mo, W, Fe, Co, Ni, Cu, Ag, Au, Pd, Pt, Bi, Sb, Sn, Pb and Ga;
xe2x80x83with
0.9xe2x89xa6axe2x89xa61;
0.16xe2x89xa6bxe2x89xa60.26;
0.01xe2x89xa6cxe2x89xa60.1;
xe2x80x83and
(2+bxe2x88x922a)/2xe2x89xa6xcex1 less than 1.5.
In this mixed ionic conductor it is preferable that M1 is at least one element selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y and Sc. More preferably, M1 is Gd and/or Y.
It is also preferable that L is at least one element selected from the group consisting of Zr, Ti, Fe, Co, Ni, Cu, Bi, Sn, Pb and Ga. More preferably, L is at least one element selected from the group consisting of Zr, Ti, Bi, Pb and Ga.
A mixed ionic conductor of another embodiment of the present invention (a second ionic conductor) includes an ion conductive oxide having a perovskite structure of the formula BaeZr1xe2x88x92zM2zO3xe2x88x92xcex2, wherein
0.9xe2x89xa6exe2x89xa61;
M2 is at least one element selected from the group consisting of trivalent rare earth elements, Bi, Ga, Sn, Sb and In;
xe2x80x83with
0.01xe2x89xa6zxe2x89xa60.3;
xe2x80x83and
(2+zxe2x88x922e)/2xe2x89xa6xcex2 less than 1.5.
In this mixed ionic conductor it is preferable that 0.16xe2x89xa6zxe2x89xa60.3. It is also preferable that M2 is at least one element selected from the group consisting of trivalent rare earth elements and In, especially elements selected from the group consisting of Pr, Eu, Gd, Yb, Sc and In.
A mixed ionic conductor of yet another embodiment of the present invention (a third ionic conductor) includes an ion conductive oxide having a perovskite structure of the formula BadZr1xe2x88x92xxe2x88x92yCexM3yO3xe2x88x92xcex3 wherein
M3 is at least one element selected from the group consisting of trivalent rare earth elements, Bi and In;
xe2x80x83with
0.98xe2x89xa6dxe2x89xa61;
0.01xe2x89xa6xxe2x89xa60.5;
0.01xe2x89xa6yxe2x89xa60.3;
xe2x80x83and
(2+yxe2x88x922d)/2xe2x89xa6xcex3 less than 1.5.
In this third mixed ionic conductor, it is preferable that M3 is at least one element selected from the group consisting of Nd, Sm, Eu, Gd, Tb, Yb, Y, Sc and In. More preferably, M3 is at least one element selected from the group consisting of Gd, In, Y and Yb.
The mixed ionic conductors of the present invention have not only the necessary conductivity for electrochemical devices such as fuel cells, but also superior moisture resistance.
Throughout this specification, xe2x80x9crare earth elementxe2x80x9d means Sc, Y, and the lanthanides (elements 57La through 71Lu). In the above formulas, xcex1, xcex2 and xcex3 are determined by the absent amount of disproportionate oxygen.
The present invention also provides devices using such a mixed ionic conductor. A fuel cell in accordance with the present invention includes as a solid-state electrolyte a mixed ionic conductor as described above. A gas sensor in accordance with the present invention includes as a solid-state electrolyte a mixed ionic conductor as described above. Using the mixed ionic conductors of the present invention provides electric devices, such as fuel cells and gas sensors, with high moisture resistance, high performance, and long lifetimes.